A customer sent us a NIB OS .10 FSR RC Engine, complete with manuals, together with some Cox engines. Although never fueled or run it has signs of being mounted (swirls around the mounting holes) and, while trying to remove the head, I buggered up two screw heads to the extent that it will be difficult to remove the screws (they are screwed in real tight).

We don't deal with OS engines, so what is the beast worth? Auction it on eBay? Anyone want it?

Just being curious, why would you take the head off of a NIB engine? Some of the older OS engines have thread pitches that are almost obsolete and difficult to source such as the MAX series. The FSR came out shortly afterwards and could also be problematic to find screws for.

Wanted to take the head off to see if it had not been run (a bit dark looking through the plug hole). Called the customer and he assured me that, although it had been mounted, it had not been run. Will be looking through my OS parts at home and thanks for price suggestion.

The older OS engines use JIS head screws. They look like Phillips head but the angles are different. This is probably why the screw head cam’d out and stripped. As already stated some of the screws used were a funky metric size and unobtanium. I can’t remember which engines used these screws. The FSR series may be new enough where the head screws are M2.5 and can be replaced with hex head cap screws. A thread pitch gauge will determine what you need. I have a couple of these engines. I’ll have to check when I’m back home later this week.

You may find that the screws are 2.6mm. My Max .15 (the one with the buggered crank) uses all 2.6mm screws. I had little trouble obtaining these in socket head, indeed I found genuine OS ones available from RC Japan. https://www.rcjapan.net/

I used a standard Phillips screwdriver to remove the screws when I cleaned and restored my OS Pet .099 engine. I buggered one of the screws to the head. Then it dawned on me that I bought a JIS screwdriver set, the one above. Lo and behold, using the #1 bit end into the ball handle, I was able to further extract that buggered screw head. I bought a set of replacement screws, then used the JIS screwdriver to install and tighten them, replacing the bad one I buggered.

Now I am sold on using JIS screwdrivers on Japanese engines. They have better grip and don't distort the screw head slots like the standard Phillips screwdrivers do.

OS still uses JIS cross recess screws. The dot indicates the JIS head as mentioned. Very old engines may have old JIS threads which had different pitches than ISO threads. Old japanese motorcycles and cars have similar problems with this as well. Thread and head type are not related. Japan adopted the ISO metric threadform in 1962, iirc. What I don't know is how long took Japan to stop using the old JIS threadform. I think .10 FSR is long after the change. OS uses M2.6 for everything. Its an unusual thread. The only other place I've even heard of it being used is photographic equipment. M2.6 is not old or JIS, it's just a thread series that's not in the preferred series. They must think there is some value in using it vs. M2.5.

gkamysz wrote:OS uses M2.6 for everything. Its an unusual thread. The only other place I've even heard of it being used is photographic equipment. M2.6 is not old or JIS, it's just a thread series that's not in the preferred series. They must think there is some value in using it vs. M2.5.

I think is more related to supplier availability. Photographic and electronics industry uses high quantities, since small JIS is readily available versus standard metric heads in the manufacturer's locale helps to maintain their profitability. It is not done for the end user convenience, it is the convenience of manufacturing. That'd be my take on it.